In recent years there has been much interest in fabrication of articles of complex shape from silicon carbide. One particular area of interest has been the forming of complex shaped articles for gas turbine engine applications from silicon carbide as this material is capable of withstanding temperatures substantially higher than the temperatures which can be withstood by present day super alloys used in gas turbine engines. When such turbine engines are operated at higher temperatures, such as the temperatures which can be used with silicon carbide components in a gas turbine engine, they become much more efficient by giving a greater amount of power for the same fuel consumption.
In attempting to form silicon carbide articles of complex shape, injection molding processes have been developed. In general, these molding processes are carried out by mixing silicon carbide particles, and optionally graphite particles, with a predetermined amount of a thermosetting binder. The article is formed in an injection molding operation, removed from the mold and subjected to heat in the absence of oxygen to reduce the thermosetting binder to carbon. The article is silicided to transform the carbon and any graphite present to silicon carbide thereby to produce a finished article of silicon carbide. A process for producing such an article is disclosed in U.S. Pat. application Ser. No. 389,770 filed Aug. 20, 1973 in the name of Douglas R. Fitchmun and assigned to the assignee of this application, which application is hereby incorporated by reference.
We have found that the prior art processes of siliciding an injection molded article containing silicon carbide and a thermosetting binder had some drawbacks. In particular, the prior art processes were slow, generally did not produce an article which was fully silicided, and were difficulty to perform on a body having any substantial thickness within a reasonable period of time.
One difficulty in the prior art is that the article manufactured in an injection molding process generally will have a slightly higher concentration of thermosetting binder at its surface. When the article is heated to pyrolize the binder, a slightly greater carbon concentration therefore develops at its surface. This extra amount of carbon can cause a closing off of the article's pore structure during a siliciding operation.